A displacement transducer used, for example in measuring the level of vacuum in the inlet manifold of an internal combustion engine, is known and is described in the specification of U.S. Pat. No. 3,942,002 which is assigned to the Assignee of the present application. This known transducer has an opaque movable member which is arranged to be moved in accordance with a parameter to be monitored. The movable member is provided with a slit arranged to pass light from two sources in relative proportions depending upon the position of the slit and hence the movable member. The light sources are driven by pulse waveforms, the pulse widths of which are modulated sequentially in a sinusoidal and a cosinusoidal manner respectively, the sinusoidal and cosinusoidal modulation being at the same frequency. The light passed by the slit impinges upon a detector which provides an appropriate signal, the phase of which is indicative of the relative proportions of light from the two sources in the light impinging on the detector, and thus indicative of the position of the slit. This signal is amplified and shaped to provide a square waveform and is then passed through an harmonic filter to reduce contamination of the signal by harmonics of the frequency of the source drive signals. The filter however introduces an undesirable phase lag and degrades waveform squareness. The signal is again squared and passed through a threshold detector for noise reduction, and is then applied to a counter arranged to count clock pulses during the period of each pulse of the signal, the resultant count, neglecting the phase error, being indicative of the phase of the signal. To eliminate the phase errors introduced by the harmonic filtering etc., one of the sources is switched so that periodically the light supplied by the two sources is in phase. The counter output during such periods is indicative of the phase error. By switching the counter at the rate of switching of the phase of the switched phase light source so that the counter alternatively increments and decrements, the count at the end of one increment and decrement cycle indicates the connected phase required.
This prior transducer suffers from several disadvantages, one disadvantage being that the transducer can only be used to monitor one displacement parameter whereas in practice, it is often necessary to monitor more than one parameter concurrently, particularly when monitoring the operating condition of an internal combustion engine. Moreover, not all of the operating parameters of an internal combustion engine can be manifested conveniently as a displacement and hence the prior transducer has a limited application and is not entirely suited to a complex operating system such as an internal combustion engine. Also, the optical displacement sensing arrangement may not be suitable for measuring all types of displacement parameters.